The art well recognizes that the tractive efforts of a vehicle are substantially improved if driving torque is applied to more than one axle. This concept underlies the employment of four wheel drive systems.
If all four wheels are positively driven by the engine, however, a severe amount of tire scraping or cornering scrub may occur as the vehicle negotiates curves or turns. Under such conditions, the front wheels must run through an arc of greater radius than that of the rear wheels, and, therefore, need to rotate faster than the rear wheels. The inherent problems such a condition creates are apparent to those skilled in the art.
One approach to solving the problems inherent in positive four wheel drive systems is to provide a manually operable jaw clutch between the drive axles thereof. The provision of a manually operable jaw clutch in the system allows disconnection of one axle from the power source. Vehicle's incorporating such manual jaw clutches are used as two wheel drive systems the vast majority of the time. As such, this system fails to provide the handling and performance benefits afforded by a full-time four wheel drive system.
Another approach to the provision of a four-wheel drive system involves the use of one or more one-way clutch mechanisms disposed between the front and rear wheel drive shafts. U.S. Pat. Nos. 4,018,317 and 4,054,065 are exemplary of such deives. These devices permit the vehicle to be transmuted from a two wheel to a four wheel drive in response to the rear drive axle overspinning or overrunning the front drive axle. In such devices, however, no provision was contemplated for full time four wheel drive. That is, these devices offer no provision for normally dividing the input torque between both drive axles under other than overspinning conditions.
Much effort has been devoted to another approach which is the provision of a center differential in a four wheel drive system. In a vehicle equipped with a center differential, the front and rear drive shafts would serve respectively to drive front and rear differentials, the shafts being powered from the engine through the center differential. Such a differential system permits overspeeding of any one or more of the wheels resulting from rounding corners or of certain wheels having a smaller effective radius than others.
Certain deficiencies, however, arise dur to the fixed torque split with this type of system. For instance, if one set of wheels should encounter a slippery or icy patch of ground and lose traction, they will spin and the differential action will cause the other set of wheels to lose driving torque. Manually operable locking means have been incorporated into such differential systems, which, when engaged, eliminate differential action between the drive shafts and positively lock the driven parts together.
Other drive systems have been proposed which combine a center differential with a clutch mechanism for imitating or simulating a four wheel drive system only when the vehicle is driven in a forward direction. One such system is shown in U.S. Pat. No. 3,627,072 issued to R. L. Smirl. The device disclosed in that patent, however, is not a full-time four wheel drive system. In fact, the device disclosed in the '072 patent allows only two of the four wheels to propel the vehicle when it is driven in a reverse direction.
Another known system has a center differential which provides an equal torque split to the front and rear drive axles of the vehicle. Instead of bevel gears, this system includes a multitude of crossed axis or worm gears to add mechanical friction. By such construction, some additional torque is transferred from the fastener to the slower turning drive axle in substantial proportion to the input torque. This system does not and cannot distinguish which axle is turning faster. Moreover, this system does not distinguish whether the difference in axle speeds results from wheel slip or by steering into a turn. Of course, when turning any additional torque transfer is detrimental to handling and can be unsafe on slippery surfaces.
A currently used system has a viscous clutch arranged in parallel with a center differential. This design uses approximately 50 or more plates in a sealed clutch housing nearly filled with a silcone fluid having a carefully selected viscosity. The clutch transfers torque from the faster to the slower turning axle in a manner roughly proportional to the speed difference between the axles. Such torque transfer is effected without distinction as to which axle may be turning faster. Moreover, such torque transfer is affected without regard to whether the speed difference is due to wheel slip or steering.
At lower axle speed differences (i.e., 2% due to tire variation), torque transfer through the viscous clutch unit slightly degrades vehicle handling and efficiency. Such degradation is usually noticable only on slippery roads.
When there is a medium level or degree of axle speed difference (i.e., 8 to 16% due to slip at one axle depending on surface conditions), torque transfer through the viscous clutch unit reaches a useful value. In this situation, the slipping wheels are at their traction limit. That is, nothing remains for lateral control of the vehicle. At this point uncontrolled wheel spin develops which results in rapid heating of the viscous clutch unit. Rapid heating of the viscous fluid reduces its viscosity and stresses the seals.
At higher axle speed differences (i.e., 20% due to turning a tight corner), torque transfer through the viscous clutch unit develops negative torque at the front axle and positive torque at the rear axle exceeding the traction limit if the vehicle is moving on ice (0.05 u) at a speed that would be safe for conventional drive (0.03 G lateral force). In this condition, no torque transfer between the drive axles is the safest even for friction based differentials, in two wheel drive vehicles.
As evidenced from a plethora of recently issued patents, more complicated externally controlled electronic systems are under development. The reaction time of such electronic systems is critical. That is, such systems must react in time to avoid breaking traction or imparting a shock load to the vehicle.
Thus, there remains a need for a simple full-time passively controlled mechanical system which allows any pair of wheels to freely overspeed or overrun the others by several percent while allowing the steerable wheels to freely overrun the other driven wheels by at least twenty percent. Moreover, such a full-time system should provide variable torque split to the front wheels ranging from under 30% to over 90%, as needed, without imparting a shock load to the drive line and before the drive wheels break traction with the road.